Clutch mechanism for use in safety apparatus

ABSTRACT

A speed-sensitive clutch device incorporates at least one drive-transmitting coupling element (17) which is connected to a safety line drum (1) so that it bodily rotates with the drum (1) about its axis of rotation and is displaceable from an inactive position (FIG. 1) into a drive-transmitting position (FIG. 2) in which it transmits drive. And the speed-sensitive clutch device also incorporates an inertia member (20) which is freely rotatable about the axis of drum rotation and yet is free to move in rotation relative to the drum itself. The inertia member (20) is formed and disposed so that on rapid acceleration of the drum (1) it exerts pressure, due to its inertia, against a said coupling element (17) thereby to cause or assist movement of the coupling element from its inactive position into its drive-transmitting position.

This invention relates to safety apparatus for use by persons working atheight. Such apparatus is of the kind which is attachable to a fixtureand incorporates a safety line drum holding a safety line for attachmentto a worker's safety harness. The safety line can be drawn from the drumin response to pulling force on the line exerted by the user in thecourse of his work but the apparatus has arresting means which functionsautomatically if the unwinding speed of the drum becomes excessive as itdoes in the event of the worker falling.

The arresting means functions to decelerate the drum, usually to rest.Thus, the clutch mechanism may operate to stop drum rotation almostimmediately, in the event of acceleration reaching a predetermined speedsuch as occurs in the event of a fall. Or, the line drum may bepermitted to continue to rotate so that the safety line may continue topay out, but at a limited, controlled speed.

Conventionally, the arresting means incorporates a centrifugal clutchdevice designed so that centrifugal force is relied upon for causingdrive-transmitting coupling elements of the clutch to move into drivingengagement with a fixed abutment to stop drum rotation; or, thedrive-transmitting coupling elements of the clutch are moved intodriving engagement with a rotatable component of a braking device, theconsequential rotation of this brake component being opposed by frictionwhich brings about the deceleration of the drum.

The conventional centrifugal clutch means hereinbefore referred to hasbeen found to be unsuitable for reliably achieving the rotationalspeed-sensitivity which some use situations require. In particular, forcertain work situations in which workers are working at a very modestheight above the ground, a worker who falls may hit the ground beforethe clutch brings into play means capable to retarding or arresting thefall, or at least before the descent speed has been reduced sufficientlyto prevent serious injury to the worker on such impact. In other words,too much time elapses between the commencement of rapid acceleration ofthe drum and engagement of the centrifugal clutch. Attempts to designthe clutch so that the centrifuging elements are actuatable bycentrifugal forces of smaller magnitude have resulted in the clutchbeing too sensitive in the sense that it is liable to be engaged byaccelerations of the drum such as may result from ordinary movements ofthe worker. This could create an additional accident hazard.

The performance of the fall-arrest or fall retard mechanism iscritically dependent on the sensitivity of the device clutch to thespeed of drum rotation and the reliability and efficiency of itsoperation. The present invention has emerged in the course of researchcarried out with a view to achieving reliability standards andspeed-sensitivity characteristics which are not attainable by apparatusincorporating a conventional form of centrifugal clutch.

Broadly stated, fall-arrest or fall-retard apparatus according to thepresent invention is characterised in that the speed-sensitive clutchfor coupling the safety line drum to another component, thereby toeffect retardation of the drum, incorporates (i) at least onedrive-transmitting coupling element which is connected to the safetyline drum so that it bodily rotates with the drum, about the axis ofrotation of the drum, and is displaceable from an inactive position intoa drive-transmitting position in which it transmits drive to said othercomponent and (ii) an inertia member which is freely rotatable aboutsaid axis and relative to the drum itself; and in that said inertiamember is formed and disposed so that in a normal operation it rotatestogether with the coupling element, while on the occurrence of rapidacceleration of the drum in consequence of a fall, the inertia memberexerts pressure, due to its inertia, against said coupling element insuch occurrances to cause or assist movement of the coupling member intoits drive-transmitting position.

The invention enables a fall to be arrested in a shorter time. In otherwords, the invention enables a fall-arrest apparatus to have a shorterresponse time (meaning a shorter time between the commencement of arapid acceleration of the safety line drum and the engagement of theclutch) than apparatus incorporating a conventional centrifugal clutch.And this result can be achieved without compromising the standard ofsafety in terms of the maximum fall-arrest force sustained by thefalling body, which latter is determined by the shock-absorbing meanswhich is necessarily present in or associated with the apparatus.

And the shortening of the response time does not necessitate making theclutch too speed-sensitive and therefore liable to cause spuriousactuation of any braking means provided for the drum during normalmovements of the worker attached to the safety line. The shorterresponse time results in fall-arrest forces being lower and this affordsthe derivative benefit that the forces imposed on the clutch are reducedand lighter gauge materials can be used. The response time depends onthe inertia of mass of the inertia member and any frictional resistanceto its rotary movement about the axis of the drum. The latter factor canbe and is preferably so small as to be negligible. In designing a givenapparatus it is a simple matter to select the mass of the inertia memberto achieve a given response time.

It is normally very desirable for the clutch to incorporate more thanone drive-transmitting coupling element and in the following furtherdescription of the invention the presence of at least two such elementswill be assumed.

It is likewise preferable for there to be only one inertia member andfor convenience reference is hereafter made to "the inertia member". Itis to be understood however that the use of more than one such member isnot excluded from the scope of the invention. For example, if there aretwo or more drive-transmitting coupling elements there may be separateinertia members for cooperating with the different coupling elements.

In certain embodiments of the present invention, the coupling elementsof the clutch are arranged so that they are movable into operativeposition under the action of centrifugal forces. For example theelements can be in the form of pivoted pawls as used in the centrifugalclutches of conventional fall-arrest apparatus as hereinbefore referredto. In such cases, the movement of the elements into operative positiontakes place under the combined action of centrifugal force and the forceexerted against such elements by the inertia member. As in conventionalcentrifugal clutches used in fall-arrest apparatus of the kind withwhich this invention is concerned, spring means can be employed forholding the elements in their inoperative positions during normalrotations of the safety line drum. The movement of the coupling elementsinto their operative positions takes place against the resistance ofsuch spring means.

The invention is particularly although not exclusively intended forensuring rapid actuation of a mechanism for braking the line drum withina relatively short time; in other words for providing apparatus whichcan be used by workers when working not very far above the ground.Accordingly, in preferred embodiments of the invention the inertia ofthe inertia member is such that on rapid acceleration of the drum suchas occurs in consequence of a fall of a worker attached to the safetyline, the drive-transmitting coupling elements of the clutch move intooperative position sooner than if reliance were placed on movement ofsuch elements solely under centrifugal force.

Another problem associated with the conventional clutch meanshereinbefore used is that of ensuring simultaneous engagement of anumber of centrifuging drive-transmitting coupling elements of theclutch. One reason for this is that at any given time gravity hasunequal effects on the movements of the different pawls. If the couplingelements do not simultaneously enter into driving engagement the clutchis subjected to driving torque eccentrically. The torque may betransmitted initially or even entirely via only one of the elements. Theeccentric forces imposed on the mechanism are liable to causesubstantial damage to the mechanism. With a view to alleviating thisproblem it has been proposed to increase the number of centrifugingelements but this expedient involves size and weight penalties on themechanism.

Preferably the coupling elements of the clutch of apparatus according tothe present invention are arranged so that in their inoperativepositions they are both in contact with or very close to a commoninertia member and this member always moves the elements simultaneouslyand to the same extent.

As described above, where braking means are provided, these willnormally be constituted by braking means which are constituted andarranged so that when they are brought into operation, the drum carryingthe safety line is first decelerated, and then brought to rest. And theinvention is applicable to apparatus incorporating such types of brakingmeans. However those skilled in the art will readily understand andappreciate that the invention is equally applicable, and indeedadvantageous, when incorporated in apparatus in which the braking meansfirst decelerates the drum, and then, instead of stopping drum rotationcompletely, allows rotation of the drum to continue, but at a controlledspeed which is sufficiently slow as to prevent or inhibit the occurranceof serious injury to a worker who is suspended on the safety line. Forexample this will permit controlled lowering of the worker towards theground or towards a platform.

This invention also extends to fall arrest apparatus comprising a linedrum supported by a shaft for rotation relative to a rigid plate memberadapted to be secured to a fixed anchorage, a safety line wound on thedrum and which can be paid out with the drum unwinding against theinfluence of a rewind coil spring, there being, operable between saidplate member and said drum, unwind rotational speed sensitive clutchmeans incorporating an inertia member of the type as above defined,operation of said clutch means as assisted by said inertia member, beingeffective substantially immediately, to stop rotation of the drumrelative to the rigid plate member when a predetermined speed of drumrotation is exceeded.

In such an arrangement where no breaking means are provided, the saidsafety line incorporates at or towards its outboard end, an energydissipating unit for cushioning shock loads occasioned in the safetyline, said energy dissipating unit being so connected in the safety linethat it can only be disconnected by such destructive breaking of theconnection as to render the connection non-reusable.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIGS. 1 and 2 are frontal views illustrating a centrifugal clutchmechanism according to this invention. In FIG. 1 the drive-transmittingcoupling elements are shown in the disengaged condition while in FIG. 2they are shown in the engaged condition;

FIG. 3 is a cross-sectional view taken on the line III--III of FIG. 1;

FIG. 4 is a side sectional elevation of one example of a fall arrest orfall retard assembly which incorporates a clutch mechanism according tothis invention;

FIGS. 5A and 5B are front elevations of the apparatus of FIG. 4 withparts removed to reveal component parts of the clutch;

FIGS. 6A and 6B are graphs comparing performances of the drum brakingmeans in apparatus with and without an inertia member.

FIG. 7 illustrates an alternative embodiment in which no brake means arepresent.

The apparatus illustrated in FIGS. 1 to 4 includes drum braking means.The apparatus comprises a cable drum 1 which is rotatable on a shaft 2.A cable 3 is wound onto the drum. The drum hub is mounted on a bearingbush 4 surrounding the shaft. An aligned bearing bush 41 also on theshaft serves as the inner bearing of a rotatable brake disc 5. Brakedisc 5 has at one side thereof an axially protruding annular rib 6 whichintrudes into an aperture in a plate 7. The rib 6 has a smooth exteriorperipheral surface and makes a close sliding fit in the said aperture sothat the plate 7 serves as an outer bearing for the brake disc. Theplate 7 constitutes a load-bearing spine which carries the drum and drumbrake assembly. The upper portion of the plate 7 is formed with anaperture 7A and serves as a handle by which the apparatus can besuspended from a fixture.

A peripheral margin of the brake disc 5 forms a radial flange 8 whichoverlaps a marginal portion of the plate 7 surrounding its saidaperture. Brake rings 9, 10 are located against the opposite faces ofthe flange 8 and these rings and the flange are held firmly together andagainst the spine plate 7 by a clamping ring 11 which is secured to thespine plate by bolts 12.

The major part of the spine plate 7 assembly which it card brakeassembly which it carries are enclosed in a casing 13. One part of thiscasing carries a centering bearing 14 for one end of the shaft 2.

In use, the cable 3 is attached to a worker's safety harness. Pullforces exerted on the cable due to normal movements of the worker causethe drum to rotate so that the necessary further length of cable isreleased and it does not restrain such movements, Such unwinding motionof the drum takes place against the action of a spiral spring 15 whichis housed in a recess in the drum and is connected at one end to thedrum and at the other end to the shaft 2. The spring servesautomatically to rotate the drum in the winding direction when windingin of the cable is not restrained by the worker. Consequently, when theworker moves nearer the place where the apparatus is secured to thefixture, the slack which would otherwise appear in the cable isautomatically taken up.

Around the inside of the rib 6 on the brake disc 5 there is a series ofabutments 16 (FIGS. 5A and 5B) which are in the form of raked teeth. Thecable drum carries coupling elements 17 which are in the form of pawlsfor engaging those teeth. The pawls are pivotally mounted on pins 18which are screwed into the drum 1. The pawls 17 are eccentricallymounted on the pins 18 so that when the drum is rotating in theunwinding direction centrifugal force tends to cause pivotal movementsof the pawls 17 such that their longer arms swing outwardly towards thesurrounding series of teeth 16. The pawls 17 are biased against suchpivotal movements by springs 19 so that they retain their inoperativepositions during slow unwinding movements of the drum such as occurduring normal pay-out of the cable. If the pawls 17 undergo sufficientpivotal movement to bring them into engagement with teeth 16 on thebrake disc 5, this disc is forced to rotate against the frictionalresistance imposed by the brake assembly comprising the spine plate 7,the brake disc 5, the clamping ring 11 and the sandwiched brake rings9,10. The frictional braking forces decelerate the drum and arrest orretard the fall.

As so far described the fall-arrest apparatus of this kind is aspreviously known. The centrifugal forces imposed on the clutch couplingelements 17 in the event of a fall are relied upon to displace them intotheir operative driving engagement with the rotatable brake disc 5. Theapparatus illustrated in FIGS. 1, 2 and 3 differs from such knownapparatus in that it incorporates an inertia member 20 which has anactivating action on the clutch coupling elements 17.

The inertia member 20 is a metal component coated with PTFE. It isfreely rotatable around the drum shaft 2, between and relative to thedrum 1 and the brake disc 5. The inertia member comprises a hub portion21 which runs on the bearing bush 4 and radial neck portions which jointhe hub portion with diametrically opposed thickened rim portions 22.FIG. 1 shows the clutch components in the positions which they normallyoccupy during use of the apparatus. The pawls 17 are in theirinoperative positions and rotate bodily with the drum 1, out ofengagement with the teeth 16 of the brake disc 5. Because the inertiamember 20 is not positively connected to the drum 1, on rotation of thedrum 1 it tends, due to its inertia, to retain its angular orientationaround the shaft 2. However the pawls 17 held in by the springs 19 abutagainst the thickened rim portions 22 of the inertia plate and cause itto accompany their rotation around the shaft. The inertia of the member20 is such that if the drum is rapidly accelerated due to a fall, thereaction force to which the pawls 17 are subjected in overcoming theinertia of the inertia member, significantly assists movement of thepawls 17 into engagement with the teeth 16 of the brake disc 5. Thepawls are displaced into engagement with teeth 16 under the combinedinfluence of that reaction force and centrifugal force and the clutch isengaged sooner than it would be if reliance were placed solely on thecentrifugal force. FIG. 2 shows the parts of the clutch in the positionswhich they occupy immediately after engagement of the drive transmittingcomponents of the clutch.

The longer arm of each of the pawls 17 abuts against one end (abutmentend) of the adjacent thickened rim portion 22 of the inertia member 20.The abutment ends of the opposed thickened rim portions 22 areaccurately diametrically opposed with respect to the axis of the inertiamember 20. Consequently the abutment forces which are exerted on the twopawls 17 due to the inertia of the inertia member 20 always act on thetwo pawls simultaneously and in balanced manner.

The thickened rim portions 22 of the inertia members are of such sizeand shape that when the pawls 17 are in their normal inoperativepositions (FIG. 1) each of those rim portions 22 is in contact with orvery nearly in contact with both of the pawls 17. Consequently there isno or virtually no angular play between the inertia member 20 and thepawls 17. If the cable drum 1 is rapidly accelerated, activating forcesexerted on the pawls 17 due to the inertia of the inertia member 20 aretherefore exerted instantanously at the commencement of any rapidacceleration of the cable drum 1.

The effect of the inertia member 20 on the braking of the cable drum andon the fall-arrest forces sustained by a falling body is illustrated bythe comparative graphs, FIGS. 6A and 6B. The graphs were plotted by aforce recorder interposed between the drum 1 of the apparatus and afixture from which it was suspended. A weight comparable to that of aheavy human body was attached to the cable 3 but was initially supportedfrom below so that it did not exert any force on the apparatus. Therecorder was set to register the suspended weight of the apparatus aszero. The weight was then released to simulate the fall of a worker.

A first test was performed with an apparatus as described with referenceto FIGS. 1 and 2 except that it did not incorporate an inertia member(member 20). FIG. 6A shows the forces transmitted to the recorder infunction of time commencing at the moment of release of the weight. Thecommencement of the initial rapid increase in the recorded force up tothe peak value F' coincides with the start of the operation of thefriction brake and took place after time T'. The fact that thesubsequent rapid reduction in the recorded force reached zero and wasfollowed by a further surge of force F2 is due to inevitable "bouncing"of the weight following the arrest of the cable drum. Persistance ofbouncing, albeit of lower amplitude, accounts for the subsequentfluctuation in the recorded force exerted by the suspended weight overthe period t2,

A second identical test was performed using fall-arrest apparatusidentical with that used in the first test except that it incorporatedthe inertia member 20. The forces recorded in function of time arerepresented by FIG. 6B. Comparing FIG. 6B with FIG. 6A it is apparentthat the inertia member had the effect of shortening the time betweenthe release of the weight and the initiation of the operation of thefriction brake from T' to t'. In consequence the kinetic energy of thefalling weight at the instant the brake was applied was considerablyless than in the case of the first test. The maximum recorded force f'was less than half of the maximum force F' recorded in the first test.

The precise effect of the inertia member on the performance of the brakemechanism in apparatus as illustrated depends on the resistance of thepawls 17 to displacement towards their operative positions bycentrifugal forces, which in turn depends, inter alia, on the biasingforce exerted by the springs 19, and on the inertia of the inertiamember 20, which is a function of its mass and the frictional resistanceto its rotational movement relative to the cable drum. In theillustrated apparatus, this latter resistance is so small as to benegligible.

While the invention has been illustrated by an embodiment in which thecoupling elements (pawls 17) of the clutch are movable towards theiroperative positions by centrifugal force the involvement of centrifugalforce in the operation of the clutch is not an essential feature of theinvention. The invention includes apparatus wherein the couplingelements are displaced into their operative positions solely under forceexerted in consequence of their abutment against the inertia member 20.For example the clutch can incorporate pivoted coupling elements whichare balanced about their pivotal axes.

Furthermore the invention is not restricted to apparatus wherein thereis a single inertia member 20. For example, the invention includes inits scope apparatus in which there are two clutch coupling elements andtwo inertia members, one for cooperating with each of such elements.

Reference will now be made to the embodiment of the invention shown inFIG. 7.

Referring to FIG. 7, the fall arrest apparatus here shown comprises aline drum 1 supported by a shaft 2 for rotation relative to a rigidplate member 7 adapted to be secured to a fixed anchorage, for examplewith an intervening swivel 74. The shaft 2 rotates in a bearing 81housed in a rigid casing 82 which is fast with the plate 7.

A safety line 3 is wound on the drum 1 and which can be paid out withthe drum unwinding against the influence of a rewind coil spring 15. Theinner end of the coil spring 15 is attached to the shaft 2 and the outerend of the spring 15 is attached to the casing 82.

An unwind rotational speed sensitive clutch means generally designated77 is operable between the rigid plate member 7 and the drum 1, and iseffective to stop rotation of the drum relative to the rigid platemember when a predetermined speed of drum rotation is reached.

The safety line 3 incorporates at or towards its outboard end, an energydissipating unit generally designated 78 for cushioning shock loadsoccasioned by the safety line. The energy dissipating unit 78 is soconnected in the safety line 3 that it can only be disconnected by suchdestructive breaking of the connection as to render the connectionnon-reusable.

For example the line 3 terminates in a loop secured by binding which canonly be undone by destructive severance. Similarly the unit 78 issecured to a metal hook member 79 and the connection therebetween issuch that it can only be unmade by destructive cutting or shearing.

The energy dissipating unit 78 is preferably of the tear webbing typewhich is per se well known. In this a length of webbing is stitched infolds and when shock occurs the stitches rupture to allow the foldedwebbing to unfold. Such devices can be set to operate at a specifiedthreshold; that is the stitches begin to rupture at a predeterminedshock loading such as 3 or 6 KN. The maximum extensibility of such adevice can be predetermined. A typical maximum extensibility is 1 metre.

The clutch mechanism 77 is a speed sensitive clutch device incorporatingan inertia member as described above with reference to FIGS. 1 to 6. Ithas pawls 17 which are engageable with teeth formed at the periphery ofan aperture 88 formed in the spine plate 7, the movement of the pawls 17into engagement with the teeth of the aperture 88 being assisted by aninertia member (not shown), the operation being analogous to theoperation of the inertia member 20 described with reference to FIGS. 1and 2.

With appropriate choice of components, design and testing, such a clutchmechanism 77 can be rendered very sensitive and lock up can be ensuredwhen a predetermined speed of drum rotation is exceeded; while on theother hand, at lower speeds, drum rotation at normal payout and rewindspeeds is freely permitted. Also, while this embodiment of the inventionprovides a device wherewith safety line payout can be interruptedabruptly whenever a predetermined payout speed is exceeded and thuspredetermination of lock up speed can be set with accuracy, moreover,once lock up occurs, the tear-webbing type of energy dissipating unitused for shock cushioning has the advantage that its maximumextensibility is known. Accordingly a fall arrest mechanism with fullypredictable performance is obtained.

Because the fall arrest mechanism now proposed and as shown in FIG. 7,has no braking mechanism, in the event of a fall, reliance is placedexclusively upon the energy dissipating unit 78, to provide cushioning,and accordingly it is vital that the unit 78 is made inseparable fromthe line 3. In this way it is not possible to make use of the line 3 anddrum 1 with its clutch mechanism 77 without the unit 78 being presentand in good operative condition. Removal of the unit 78 is only possibleby destructive severance of its connections. If the unit 78 is of thewebbing type it breaks open when used, giving a clear indication of suchuse.

Similarly the line 3 will be attached to the line drum 1 by connectingmeans which require to be broken if the attachment is to bediscontinued. Thus the drum 1, the line 3, the device 78 and theterminal hook member are all inseparably connected together and must bereplaced all together. There is therefore little likelihood ofinadvertent reuse which might involve risk. The intention is that thedevice now proposed should be used once only, and that after such singleuse it will require to be entirely replaced.

Although the energy dissipating unit 78 inseparably incorporated in thefall apparatus of the present invention, is preferably of the abovedescribed tear webbing type, other types of energy dissipating unit canbe employed instead, provided that they are incorporated by means ofconnections which can only be unmade by destructive severance.

Also as above described the drum 1 is supported by the shaft 2 forrotation relative to a rigid spine plate member 7 by means of a bearing81 in a casing 82 fast with the plate 7. However, in apparatus accordingto the invention, instead of the spine plate 7 with its casing 82, therecould be substituted a rigid housing. In this case, the drum 1 would belocated encased within the housing and the shaft 2 could have itsopposite ends supported by bearings fixed to the housing walls.

As a further alternative, the fall arrest apparatus according to theinvention could be provided with a manually operable winch mechanismwith the aid of which the line drum could be wound or unwound. Forexample such a winch might be employed to lower a fallen worker to aplatform or to the ground.

I claim:
 1. Fall-arrest safety apparatus which is attachable to afixture and incorporates a safety line drum (1) for holding a safetyline (3) and permitting it to be drawn from the drum (1) in response topulling force on the line exerted by a person attached to that line; theapparatus having means which functions automatically initiated byactivation of a speed sensitive clutch device to decelerate the drum ifits unwinding speed becomes excessive as it does in the event of theworker falling; characterised in that the speed-sensitive clutch devicefor initiating deceleration of the safety line drum incorporates (a) atleast one drive-transmitting coupling element (17) which is connected tothe safety line drum (1) so that it bodily rotates with the drum (1)about its axis of rotation and is displaceable from an inactive position(FIG. 1) into a drive-transmitting position (FIG. 2) in which ittransmits drive and (b) an inertia member (20) which is freely rotatableabout said axis and relative to the drum itself; and in that saidinertia member (20) is formed and disposed so that on rapid accelerationof the drum (1) in consequence of a fall it exerts pressure, due to itsinertia, against said coupling element (17) thereby to cause or assistmovement thereof into its drive-transmitting position.
 2. Apparatusaccording to claim 1, wherein said at least one coupling element (17) isarranged so that it is movable into operative position (FIG. 2) underthe action of centrifugal forces and the movement of said element (17)into operative position takes place under the combined action ofcentrifugal force and pressure exerted by the inertia member (20). 3.Apparatus according to claim 2, wherein the inertia of the inertiamember (20) is such that on rapid acceleration of the drum (1) such asoccurs in consequence of a fall of a worker attached to the safety line,the coupling elements (17) of the clutch move into operative positionsooner than if reliance were placed on movement of such elements (17)solely under centrifugal force.
 4. Apparatus according to claim 1,wherein there is more than one said coupling element (17) and whereineach said coupling element is in contact or very close to a commoninertia member (20) which always moves the plural elements (17)simultaneously and to the same extent.
 5. Apparatus according to claim1, and in which the drum (1) is carried on a shaft (2) for rotationrelative to a fixed plate (7) adapted to be secured to an anchorage atan elevated location on a building structure or the like, and in whichthe drive transmitting coupling element (17) is in the form of a pawlwhich is mounted on the drum (1) for pivotal movement relative to thedrum, said pawl moving within an aperture (88) in said plate (7) andsaid pawl (17) being pivotal into engagement with a tooth formed at theperiphery of said aperture (88) in said plate (7) in order to stoprotation of said drum (1) relative to the plate (7).
 6. Apparatusaccording to claim 5, and wherein the safety line (3) wound on the drum(1) incorporates at or towards its outboard end, an energy dissipatingunit (78) for cushioning shock loads occasioned in the safety line (3),said energy dissipating unit (78) being so connected in the safety line(1) that it can only be disconnected by such destructive breaking of theconnection as to render the connection non-reusable.
 7. Apparatusaccording to claim 1, and in which the drum (1) is carried on a shaft(2) for rotation relative to a fixed plate (7) adapted to be secured toan anchorage at an elevated location on a building structure or thelike, and in which the drive transmitting coupling element (17) is inthe form of a pawl which is mounted on the drum (1) for pivotal movementrelative to the drum, said pawl (17) being pivotal into engagement witha rotatable braking component (6, 8), there being a non-rotatablebraking component (11) fast with said plate member (7) and frictionbrake means (9, 10) interposed between said rotatable and saidnon-rotatable breaking components effective to resist relative rotationof said respective components, all whereby rotation of said drum (1)relative to said plate (7) is retarded when said pawl (17) pivots intoengagement with the rotatable break component (6, 8).